QPaintEvent in MainWindow

Donn

I'm still confused... Sorry for my stupid questions but I want to make it clear. I assume MyLabelWithOverlay inherits most of the methods provided by QLabel and I don't get why it cannot directly display text with label->setText() ?

kshegunov

@Donn
It inherits all of the methods provided by QLabel, but it overrides the QLabel::paintEvent method, so no painting is done from QLabel::paintEvent and consequently you don't get the text painted (which is what QLabel::paintEvent actually does).

Donn

So QLabel::paintEvent is actually responsible for painting a simple text and if I override it doesn't provide the functionality for displaying text anymore and I need to do it myself in the body of overridden function, right? Can you provide some code how it could be in the body because I don't have any idea.

kshegunov

@Donn said:

So QLabel::paintEvent is actually responsible for painting a simple text and if I override it doesn't provide the functionality for displaying text anymore and I need to do it myself in the body of overridden function, right?

Correct.

Can you provide some code how it could be in the body because I don't have any idea.

void MyLabelWithOverlay::paintEvent(QPaintEvent * event)
{
    // This code draws the pixmap that was given with setPixmap(), and hopefully it does that correctly 
    QPixmap pix = pixmap();
    qreal ratio = qMin(static_cast<qreal>(width()) / pix.width(), static_cast<qreal>(height()) / pix.height());

    QTransform transform;
    transform.scale(ratio, ratio);

    QRect sourceRect = transform.mapRect(event->rect());

    QPainter painter(this);
    painter.drawPixmap(event->rect(), pix, sourceRect); //< Pixmap was drawn

    if (overlayEnabled)  {
        //< Here you can draw on top of that pixmap the lines (that's why it was left unfilled)
    }

    // Here you can draw the text (on top of the pixmap and the lines
    QRect widgetRect = rect(); //< This is the widget's rectangle
    QString labelText = text(); //< This is the text that was set
    // This following line draws the text centered horizontally and bound by the provided rectangle
    painter.drawText(widgetRect, labelText, Qt::AlignHCenter); 
}

Donn

void MyLabelWithOverlay::paintEvent(QPaintEvent * event)
{
    const QPixmap pix = *pixmap();
    qreal ratio = qMin(static_cast<qreal>(width()) / pix.width(), static_cast<qreal>(height()) / pix.height());

    QTransform transform;
      transform.scale(ratio, ratio);

    QRect sourceRect = transform.mapRect(event->rect());

    QPainter painter(this);
    painter.drawPixmap(event->rect(), pix, sourceRect);

    if (overlayEnabled)  {
        // Draw your lines here
    }
    // Here you can draw the text (on top of the pixmap and the lines
    QRectF widgetRect = rect(); //< This is the widget's rectangle
    QString labelText = text(); //< This is the text that was set
    // This following line draws the text centered horizontally and bound by the provided rectangle
    //painter.drawText(widgetRect, labelText, Qt::AlignHCenter);
}

My app crashes after this saying that

QWidget::repaint: Recursive repaint detected

If I uncomment the last line I get the following:

C2664: 'void QPainter::drawText(const QRectF &,const QString &,const QTextOption &)': cannot convert argument 1 from 'QRectF' to 'int'

kshegunov

const QPixmap pix = *pixmap();

Since you're not checking if the pointer returned is NULL, have you set a pixmap for the label with QLabel::setPixmap?
If you haven't this would cause a segfault, because dereferencing a NULL pointer isn't allowed.

Recursive repaint detected

This shouldn't be normally happening. Are you calling repaint() manually, or is there an explicit call to paintEvent() somewhere?

If I uncomment the last line I get the following: ... cannot convert argument 1 from 'QRectF' to 'int'

Have you included the header for QRectF?

jsulm

@kshegunov Wouldn't it be easier to first call QLabel::paintEvent(...) to get back the inherited functionality and then draw custom stuff?

Donn

@kshegunov said:

Since you're not checking if the pointer returned is NULL, have you set a pixmap for the label with QLabel::setPixmap?
If you haven't this would cause a segfault, because dereferencing a NULL pointer isn't allowed.

Could you please provide a solution for this because I really don't get it how it is done since it is a big mess for me and trying to get it step by step. I really appreatiate your help!

This shouldn't be normally happening. Are you calling repaint() manually, or is there an explicit call to paintEvent() somewhere?

I do not call any repaint() or paintEvent() explicitly I think. I just simply created a header file and a source file as mentioned above and have included header file to other header file and have promoted one QLabel widget to MyLabelWithOverlay.

Have you included the header for QRectF?

I have included the header but it is still the same problem.

kshegunov

@jsulm
Absolutely! Although, I was half way down writing the code when I realized I had a QLabel instead of QWidget (which would be usually the case) and was reluctant to rewrite it. :)

@Donn

Could you please provide a solution for this because I really don't get it how it is done since it is a big mess for me and trying to get it step by step.

Solution for what you mean? How to check whether a pointer is null? Simply check with if:

QPixmap * pixmapPointer = pixmap();
if (!pixmapPointer)
    return;

QPixmap & pix = *pixmapPointer; //< You have to make sure pixmapPointer points to a valid object before using the deref operator (the *)

Also something I noticed in your code is that you're passing const QPixmap * to QPainter::drawPixmap, which accepts const QPixmap &, you should fix that (look the snippet how conversion between the two is done).

I have included the header but it is still the same problem.

Usually this happens when the header is not present. Try passing the object directly:

painter.drawText(QRectF(rect()), labelText, Qt::AlignHCenter);

Donn

@kshegunov

Everything mentioned above compiles except the last problem:

Usually this happens when the header is not present. Try passing the object directly:
painter.drawText(QRectF(rect()), labelText, Qt::AlignHCenter);

I still get the same error message. Instead I tried

painter.drawText(20, 20, labelText);

which compiles but I don't get any text displayed in the promoted MyLabelWithOverlay label.

EDIT:

Maybe I'm doing something wrong in other place. If you can, please check it because I have no idea what could be wrong. Perhaps somewhere I don't completely get how things work...

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

MainWindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "mylabelwithoverlay.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->label_5->setText("hello world"); // label_5 is promoted to MyLabelWithOverlay
}

MainWindow::~MainWindow()
{
    delete ui;
}

mylabelwithoverlay.h:

#ifndef MYLABELWITHOVERLAY_H
#define MYLABELWITHOVERLAY_H

#include "QtGui"
#include "QtCore"
#include "QLabel"


class MyLabelWithOverlay : public QLabel
{
    Q_OBJECT

public:
     MyLabelWithOverlay (QWidget * parent = NULL);

public slots:
     void enableOverlay(bool enabled);

protected:
     virtual void paintEvent(QPaintEvent * event);

private:
     bool overlayEnabled;
};


#endif // MYLABELWITHOVERLAY_H

MyLabelWithOverlay.cpp:

#include "mylabelwithoverlay.h"
#include <QRect>
#include <QRectF>


MyLabelWithOverlay::MyLabelWithOverlay (QWidget * parent): QLabel(parent), overlayEnabled(false)
{
}

void MyLabelWithOverlay::enableOverlay(bool enabled)
{
    overlayEnabled = enabled;
}


void MyLabelWithOverlay::paintEvent(QPaintEvent * event)
{
    const QPixmap * pixmapPointer = pixmap();
    if (!pixmapPointer)
        return;

    const QPixmap & pix = *pixmapPointer;

    qreal ratio = qMin(static_cast<qreal>(width()) / pix.width(), static_cast<qreal>(height()) / pix.height());

    QTransform transform;
      transform.scale(ratio, ratio);

    QRect sourceRect = transform.mapRect(event->rect());

    QPainter painter(this);
    painter.drawPixmap(event->rect(), pix, sourceRect);

    if (overlayEnabled)  {
        // Draw your lines here
    }
    // Here you can draw the text (on top of the pixmap and the lines
    QRect widgetRect = rect(); //< This is the widget's rectangle
    QString labelText = text(); //< This is the text that was set
    // This following line draws the text centered horizontally and bound by the provided rectangle
    //painter.drawText(QRectF(rect()), labelText, Qt::AlignHCenter);
    painter.drawText(20, 20, labelText);

}

kshegunov

@Donn
You're not setting a pixmap for the label, so when the paint events gets executed pixmapPointer will be NULL and the function will return. If you wish to support both/either text and pixmap you have to enclose each in its own if block.

void MyLabelWithOverlay::paintEvent(QPaintEvent * event)
{
    QPainter painter(this);

    const QPixmap * pixmapPointer = pixmap();
    if (pixmapPointer)  {
        const QPixmap & pix = *pixmapPointer;
        qreal ratio = qMin(static_cast<qreal>(width()) / pix.width(), static_cast<qreal>(height()) / pix.height());

        QTransform transform;
        transform.scale(ratio, ratio);

        QRect sourceRect = transform.mapRect(event->rect());
        painter.drawPixmap(event->rect(), pix, sourceRect);
    }

    if (overlayEnabled)  {
        // Draw your lines here
    }

    QString labelText = text();
    if (!labelText.empty())  {
        painter.drawText(0, 0, labelText);
    }
}

Donn

@kshegunov

I think I got it! I have included to the painterEvent the following:

QPixmap pm(200, 200);
pm.fill(Qt::white);
setPixmap(pm);

Now I can set the text from other class like:

ui->label->setText("lala");

However, I think I shouldn't set Pixmap inside paintEvent - better to do it in the other class so this type of label could be also used for other labels, right?

EDIT:

I liked the idea of @jsulm. Could you give me some hints how to achieve it? How to get the painting functionality from a standard QLabel and then add some additional my own painting?